Negative Mitzvah 355
It is forbidden to take a woman as a wife without marriage
Deuteronomy 23:18 "There shall be no indecent women among the daughters of Israel"
A Jewish marriage must be held according to the law of the Torah.
This includes the entire wedding ceremony with all its details and procedures and the signing of the marriage contract.
A person is forbidden to take a woman as a wife without fulfilling these requirements.